1. Field of the Invention
The present invention relates to an imaging lens and an imaging device including the imaging lens. In particular, the present invention relates to an imaging lens and an imaging device including the imaging lens, in which the imaging lens has a three-lens structure that is capable of size and weight reduction. The imaging lens is used in an image-taking device that uses an image sensor element, such as a charge-coupled device (CCD) and a complementary metal oxide semiconductor (CMOS), mounted on a portable computer, a television phone, a portable phone, and the like.
2. Description of the Related Art
In recent years, there has been an increasing demand for cameras that utilize an image sensor element, such as the CCD, the CMOS, or the like, that is mounted on a portable computer, a television phone, a portable phone, a digital camera, and the like. It is demanded that a camera such as this is small and light because the camera is required to be mounted on a limited installation space.
Therefore, it is also necessary for the imaging lens used in such cameras to be similarly small and light. Conventionally, a single-lens structure lens system using a single lens and a two-lens structure lens system having two lenses have been used as such an imaging lens.
However, although such lens systems with a small number of lenses are extremely effective in reducing the size and weight of the lens system, the lens systems cannot sufficiently handle high image quality and high resolution required of the imaging lenses in recent years.
Therefore, conventionally, a three-lens structure lens system using three lenses is used to handle high image quality and high resolution. The three lenses are a first lens, a second lens set on an object surface side of the first lens, and a third lens set on the object surface side of the second lens.
As examples of such a three-lens structure lens system attempting to increase resolution, for example, the lens systems described in the following Patent Literatures 1 to 3 are known.
[Patent Literature 1] Japanese Patent Unexamined Publication 2001-75006
[Patent Literature 2] Japanese Patent Unexamined Publication 2003-149545
[Patent Literature 3] Japanese Patent Unexamined Publication Heisei 10-301021
However, in the lens system described in Patent Literature 1, a diaphragm is disposed closest to the object side. Therefore, correction of distortion and chromatic aberration caused by magnification is difficult. A first lens is a meniscus lens whose convex surface faces an image surface side. Therefore, the lens system is not suitable for size and weight reduction. A second lens is shaped having a concave surface on the image surface side. Therefore, total reflection easily occurs and correction of off-axis aberration is difficult.
In the lens system described in Patent Literature 2, a first lens has negative power. Therefore, size and weight reduction is difficult.
Furthermore, in the lens system described in Patent Literature 3, a diaphragm is disposed between a second lens and a third lens. The third lens has a strong concave surface facing an object side and negative power. Therefore, telecentricity is poor. Moreover, an effective diameter of the third lens is large because space between the second lens and the third lens is wide. Therefore, the lens system is not suitable for size and weight reduction.
Therefore, the conventional lens systems were insufficient for securing telecentricity and achieving excellent resolution through sufficient correction of various aberrations, while achieving size and weight reduction.